A recent trend in the field of analytical instrumentation has been the development of integrated microfluidic devices in which multiple operations are performed on a single device, e.g., Harrison et al., “Micromachining a Miniaturized Capillary Electrophoresis-Based Chemical Analysis System on a Chip,” Science, 261: 895 (1992). Such devices offer many advantages over conventional analytical formats including the ability to handle very small volumes; ease and economy of device fabrication; the ability to integrate multiple operations onto a single integrated device; and the opportunity to achieve a high degree of automation.
In many chemical and biochemical analysis methods performed using microfluidic devices, it is advantageous to concentrate an analyte as part of the analysis. For example, increased analyte concentration generally leads to increased chemical reaction rates, increased rates of mass transfer, and enhanced detectability. However, because conventional concentration methods require a solid phase pullout step (e.g., adsorption), or a phase change of the analyte (e.g., precipitation), or a phase change of the solvent (e.g., evaporation), these methods are not well adapted for use in a microfluidic device.
In addition, methods for controlling the location of an analyte are important in the design of methods using microfluidic devices. For example, prior to a separation step, it may be desirable to locate a sample volume in a spatially-defined injection zone.
Therefore, it would be desirable to have a method for the location and concentration of an analyte that is well suited for use in integrated microfluidic systems.